The invention relates to improvements in inductively operated heating apparatus, and more particularly to improvements in apparatus which can be utilized with advantage to heat plastic materials, e.g., to heat granular and/or pulverulent plastic materials in extruders and like machines. Apparatus of the type to which the present invention pertains can be used to heat conveyed particles of plastic material, for example, to form mixtures of different plastic materials or to form new mixtures of certain plastic materials.
An inductively operated heating apparatus of the above outlined character normally comprises a housing which is made of steel so that the magnetic field can establish therein eddy currents which raise the temperature of the housing. The magnetic field is generated by one or more laminated iron cores and one or more exciting coils or windings. Each core has legs which are magnetically coupled to (i.e., they normally abut or are closely adjacent) the housing so that the latter completes the path for magnetic force lines. As a rule, each core resembles the letter E or the letter U.
It is further known to prevent overheating of the exciting coils or windings. In accordance with a known proposal, the coil is hollow to define an internal path for the flow of a coolant.
A drawback of conventional inductively operated heating apparatus of the above outlined character is that they are incapable of heating the housing to an elevated temperature (as a rule, the maximum achievable temperature will be in the range of 200.degree. C.) or that the exciting coil must be operated as a low voltage coil in conjunction with an additional transformer. On the other hand, certain recently developed types of plastic material must be processed at elevated temperatures (ranging up to and even in excess of 500.degree. C.) Conventional exciting coils (which are made of an ideal conductive material, such as copper) are likely to be destroyed at such temperatures, i.e., by heat which is transmitted to the exciting coil or coils by the housing through the medium of the core or cores.